1. Field of the Invention
The present invention relates to digital circuits. More particularly, the present invention relates to digital circuits having a low voltage portion and a high voltage portion between which it is desired to pass digital signals.
2. Description of the Prior Art
It is known in the field of digital circuits to provide digital circuits with an interface circuit between a low voltage portion and a high voltage portion. FIGS. 1A, 1B, 2A and 2B of the accompanying drawings show examples of such interface circuits.
FIGS. 1A and 1B show an example of an interface circuit taking the simple form of an invertor 2 composed of a p-type transistor 4 and an n-type transistor 6. The signal from the low voltage portion is first passed to a buffer in the form of an invertor 8 that serves to ensure the digital signal being passed to the high voltage portion is either at the ground voltage level or at the low voltage supply level Vdd-low. When the digital signal input to the transistors 4, 6 is a "0", as shown in FIG. 1A, then transistor 4 is fully switched on whilst transistor 6 is fully switched off leading to an output value of "1" with substantially no dc current flowing through the transistors 4,6.
FIG. 1B shows the situation in which the signal applied to the inputs of the transistors 4, 6 is the low voltage supply level Vdd-low. This input signal is insufficient in level to filly switch off transistor 4 or fully switch on transistor 6. As a result the desired output of "0" is not reliably produced since the transition voltages of the transistors 4,6 may not be adequately cleared by the input signal level. Furthermore, a dc current path is present through the partially switched on transistors 4,6. This dc current flow is a severe disadvantage since it wastes a considerable amount of power.
FIG. 2A illustrates a modified interface circuit using a pull up resistor 10 in place of the p-type transistor 4 of FIGS. 1A and 1B. In the case illustrated in FIG. 2A, the input value to the interface circuit 2 is a "0" and again fully switches off the transistor 6 producing an output value of "1" having the full high voltage supply level Vdd-high. In the case illustrated in FIG. 2B, the transistor 6 is again not completely switched on by the signal Vdd-low, but in this case the value of the resistor 10 can be chosen to be significantly higher than the resistance of the transistor 6 in this partially switched on state such that the output from the interface circuit 2 is reliably at a level of "0" in this circumstance. (The resistor element may be a tied of transistor or alternatively could a weak p-type transistor sharing its gate input with the n-type transistor.) However, the problem of the dc current path still persists and is a severe disadvantage.
In modern digital circuits it is desired to run different portions of the the circuit at different voltage levels to save power, but the number of signals that must be passed from the low voltage portion to the high voltage portion can be very large and so the number of interface circuits will be correspondingly large. If each of these interface circuits wastes power through the above dc current, then the total power loss is significant.